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A

7/03

DI-53

Table 1. Transformer Design Parameters.

Figure 2. Efficiency vs. Output Power.

Key Design Points

• Capacitors C20, C

reset. Choose values to ensure sufficient reset at low line

and safe maximum drain voltage at high line. Also use

300 kHz operation for longest reset time.

• Capacitors C21 and C22 will capacitively drive MOSFET

gate capacitances C

C22 should be chosen to ensure that gate drive voltage

attains turn-on threshold of MOSFET (Vg

conditions (low line for forward MOSFET).

• Reduce transformer leakage inductance by filling each

winding layer across the entire width of the bobbin.

• Higher efficiency (+1%) can be acheived by using a

DPA426R and increasing R3 to reduce the internal current

limit.

26

8

410

12

75

65

70

60

80

85

95

90

36 VDC

54 VDC

72 VDC

TRANSFORMER PARAMETERS

Core Material

Bobbin

Winding Details

5 V (6, 7-8, 9), Primary (1-10), 3.3 V

(4.5-2.3)

Winding Order and

Pin Numbers

Primary Inductance

Primary Resonant

Frequency

Leakage Inductance

Primary 11T, 4 x 28 AWG

3.3 V 2T, 2 x 4 x 26 AWG

5 V 3T, 2 x 4 x 26 AWG

3.8 MHz (minimum)

Ferroxcube P/N: EFD25-3F3, ungapped

10-pin EFD25 surface mount bobbin

250 µH ±25% at 300 kHz

0.8 µH (maximum)

INDUCTOR PARAMETERS

Core Material

Bobbin

Winding Details

Inductance

Pin (1, 2-3, 4): 3.5 µH ±10% at 300 kHz

5 V (9, 10-7, 8), 3.3 V (1, 2-3, 4),

Bias (FL1-FL2)

5 V 6T, 2 x 4 x 26 AWG

3.3 V 4T, 2 x 4 x 26 AWG

Bias 12T, 1 x 30 TIW

10-pin EFD20 surface mount bobbin

Ferroxcube P/N: EFD25-3F3 ungapped

Winding Order and

Pin Numbers

Table 1. Inductor Design Parameters.